Existing Complementary Metal-Oxide-Semiconductor (CMOS) imaging sensors (CIS) designed for low light level visible light and/or Near-Infrared (NIR) applications are typically limited in sensitivity by the shot noise on the detector's dark current and the thermal noise of the pixel source follower. These limitations stem from the design of the electronic circuits that make up the image sensors pixels themselves.
Current visible/NIR CIS pixels, which are almost exclusively silicon-based, utilize pinned photo-diodes (PPDs) and Field-Effect Transistors (FETs) to convert the light to electrical signals and perform in-pixel signal processing. These pixels have been either nMOS based utilizing a p-type substrate/well or pMOS based utilizing an n-type substrate/well. nMOS based pixels use n-channel metal-oxide semiconductors (nMOSFET, or nFET) and n-on-p substrate photo-diodes, or, less commonly and only more recently, pMOS based pixels use p-channel metal-oxide semiconductors (pMOSFET, or pFET) with p-on-n substrate photo-diodes. Neither solution, however, is ideal for obtaining the lowest possible noise.
These pixel designs are driven by the commercial imaging market, especially the market for small, high-megapixel imaging sensors, which are commonly used in mobile devices. The aforementioned markets place great emphasis on constructing ever smaller pixels, as opposed to pixels capable of collecting the most light and producing the least noise for low visible light applications. Unfortunately, such small sensors cannot afford the extra real estate required to implement a true CMOS pixel solution, i.e. one that contains both nMOS and pMOS components, forcing developers to choose between one of the two single-flavor pixel technologies. Unfortunately, either choice leads to pixels with high noise for low light level imaging applications.
More specifically, a pMOS pixel exhibits lower dark current (by a factor of 2-4) and therefore the shot noise associated with this dark current is also lower, compared to an nMOS pixel. The pMOS pixel source follower, however, has a lower mobility (⅓ to ¼) and lower transconductance as compared to an nFET follower. The transconductance refers to the gain of a FET, which is the ratio of the change in current at the output terminal to the change in voltage on the input gate. Transconductance can be increased by raising the operating current of the device. The thermal noise of a FET is inversely related to the square-root of its transconductance. As a consequence of these characteristics, a pFET source follower has relatively high thermal noise compared to an nFET source follower operating at the same current and power.
In comparison, nMOS-based pixels have lower thermal noise from the source follower but suffer from higher shot noise from increased dark current of the photo-diode. With these limitations in mind, neither an all pMOS nor an all nMOS, i.e. “single flavor”, design can achieve both low dark current shot noise and low source follower thermal noise.
What is needed, therefore, is a pixel design that combines the benefits of each single-flavor designs to achieve lower noise and higher sensitivity. Preferably, such a solution would reduce the overall pixel noise by a factor of the square-root of 2 or more and improve the sensitivity of low light level/NIR CIS beyond those currently available.